In California, mobile vaccine clinics in rural areas were canceled. In Chicago, vaccination events for restaurant employees and aviation workers were postponed indefinitely. And at colleges in Ohio, New York and Tennessee, where the one-dose vaccine offered a chance to quickly inoculate students before they left campus for the summer, appointments were called off en masse. “It really fit into this kind of tight deadline we were doing,” Ms. Tirone said of the Johnson & Johnson vaccine. She said she hoped to offer another on-campus clinic in Youngstown with one of the two-dose alternatives, but she worried that students would not be eligible for a second dose until finals week or later... But in some places, there was no immediate alternative. In Aurora, Ill., a mass vaccination clinic planned for Tuesday was called off, leaving 1,000 patients without appointments. In Riverside County, Calif., mobile clinics that had planned to vaccinate about 400 people in less populous areas on Tuesday were canceled. And in rural Jefferson County, in southeast Iowa, a Johnson & Johnson clinic targeting manufacturing workers was scrapped at the last minute. “It was so heartbreaking to me,” said Christine Estle, the county’s public health nurse administrator, who said she and her colleagues had encouraged the roughly 140 people scheduled to attend to make appointments at local pharmacies or hospitals.